Remove one use of IDENT_USERNAME_MAX

Remove one use of IDENT_USERNAME_MAX

IDENT_USERNAME_MAX is the maximum length of the information returned
by an ident server, per RFC 1413.  Using it as the buffer size in peer
authentication is inappropriate.  It was done here because of the
historical relationship between peer and ident authentication.  To
reduce confusion between the two authenticaton methods and disentangle
their code, use a dynamically allocated buffer instead.

Discussion: https://www.postgresql.org/message-id/flat/c798fba5-8b71-4f27-c78e-37714037ea31%402ndquadrant.com